watchdog: diag288_wdt: fix __diag288() inline assembly

The DIAG 288 statement consumes an EBCDIC string the address of which is
passed in a register. Use a "memory" clobber to tell the compiler that
memory is accessed within the inline assembly.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>